This disclosure relates to passive starting strategies, and more particularly, to passive starting strategies for a Torque-Ripple-Free Electric Power Steering system.
Electric power steering (xe2x80x9cEPSxe2x80x9d) has been the subject of development by auto manufacturers and suppliers for over a decade, due in part to its potential advantages of fuel economy and ease-of-control when compared with traditional hydraulic power steering (xe2x80x9cHPSxe2x80x9d). However, commercialization of EPS systems has been slow and is presently limited due to cost and performance challenges. Among the most challenging technical issues are the pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet steering requirements.
Despite the relatively low levels of torque ripple and noise of EPS systems using conventional PM brushless motors, they are no match to the smoothness and quietness of HPS, with its long history of performance refinement efforts. Consumers are reluctant to compromise the refined features of HPS. Several performance and cost issues have stood in the way of broad-based EPS commercialization regardless of the technology used.
Upon power-up of a typical EPS system, it is generally impossible to determine the rotor position of the EPS motor by using incremental encoder signals until the motor is moved across an index position for the first time. Thus, until the EPS motor crosses the index position, the motor position is estimated by using three additional low-resolution commutation sensors to provide initial position information.
In an EPS system, it is desirable to reduce the size and cost of the encoder and sensors. The typical encoding scheme uses at least one encoding track for each of two high-resolution quadrature pulse-trains. In addition, at least three tracks are used foe low-resolution commutation sensor signals, which are used for position estimation. It is desirable to reduce the size and cost of the encoder by reducing the number of tracks required on the encoder.
Typically, the commutation sensors in an EPS system are used only once, immediately following EPS system power-up. The commutation sensors provide some information that helps to provide an assisted torque for starting an EPS motor until the rotor reaches an indexed position. Once the EPS system is synchronized to the indexed position, precise position information becomes available and the commutation sensors are no longer needed. Various methods for starting PM motor drives have certain limitations, such as either the limitation of:causing the EPS motor to rotate in an arbitrary direction before it locks to the index pulse or the limitation of requiring the use of current sensors and complicated algorithms. It is desirable to provide a method for starting a torque-ripple-free (xe2x80x9cTRFxe2x80x9d) EPS system that would preferably operate without costly and generally inaccurate commutation sensors, and which would preferably eradicate the sources of torque-ripple under ideal conditions, as well as reduce the noise level of the TRF EPS system.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for passively starting an electric power steering (xe2x80x9cEPSxe2x80x9d) system.
In an exemplary embodiment, a method for passively starting an EPS system includes disabling assist torque upon power-up, detecting an index pulse upon a movement of a steering column across one of a plurality of reference positions, enabling assist torque for the EPS system upon detecting the index pulse, receiving a variable input torque command indicative of an external steering input, and ramping a variable assist motor torque towards the variable input torque command after enabling assist torque for the EPS system, until the variable assist torque is substantially equal to the variable input torque command.
An exemplary EPS system apparatus includes a steering wheel for receiving operator inputs coupled to a steering column, a motor coupled to the steering column for applying torque assist to the column, an encoder coupled to the motor for determining the angular displacement of the steering column where the encoder comprises at least one track (e.g., two tracks) and a set of sensors for sensing the at least one track to generate a set of signals, a controller connected in signal communication between the encoder and the motor for receiving the set of signals from the encoder and controlling the motor in correspondence with a method for passively starting the electric power steering system. The controller comprises an index pulse detector, an enabler responsive to the index pulse detector for enabling an amplifier, and an amplifier for ramping the variable assist torque in correspondence with a variable input torque command. The variable input torque command may correspond to an operator input or to an automatic control signal, such as for hands-free operation.